Kinetics and modeling of anaerobic digestion process

Enhancing the resistance to H2S toxicity during anaerobic digestion of low-strength wastewater through granular activated carbon (GAC) addition

Low-strength wastewater was treated using two laboratory-scale up-flow anaerobic sludge blankets (UASB) for 130 days under sulfate-reducing conditions. Granular activated carbon (GAC) was added to one of the reactors. The GAC addition increased the total chemical oxygen demand removal by 21-28% and total methane production by 32-78%. The sludge from the GAC-amended UASB showed higher specific methanogenic activities (SMA) and higher activities in the presence of H₂S, indicating that the GAC addition enhanced the resistance of methanogens to H₂S toxicity. Further, the microbial communities showed that the GAC addition shifted microbial communities. A robust syntrophic partnership between bacteria (i.e., Bacteroidetes_vadinHA17 and Trichococcus) and methanogens was established in the GAC-amended UASB. Sulfate-reducing bacteria (SRB) were enriched in the GAC biofilm, indicating the coexistence of competition and cooperation between SRB and methanogens. These findings provide significant insights regarding microbial community dynamics, especially SRB and methanogens, in a GAC-amended anaerobic digestion process under sulfate-reducing conditions.

Archaeal community dynamics in biogas fermentation at various temperatures assessed by mcrA amplicon sequencing using different primer pairs

In this study, the taxonomic and functional diversity of methanogenic archaea in two parallel 120 l fermenters operated at different temperatures and fed with maize silage was estimated by mcrA metabarcoding analysis using two typical primer pairs (ML and MLA) amplifying part of the functional methyl coenzyme M reductase (mcrA) gene. The alpha diversity indices showed that the ML primer pair detected a higher Operational Taxonomic Unit (OTU) abundance compared to the MLA primer pair and methanogen diversity was significantly lower in the 60 °C fermenters. The beta diversity analysis showed the methanogenic community clustered together at 50 °C and 40° and was statistically different from the 60 °C community. Similar, to alpha diversity, beta diversity was also significantly different between primer pairs. At all temperatures analysed, the primer pairs showed a different abundance of the different methanogenic OTUs, e.g. more OTUs relative to Methanoculleus sp. with the ML primer pair, and more OTUs corresponding to Methanobacterium sp. with the MLA primer pair. Moreover, OTUs corresponding to Methanosphaera sp. and Methanobrevibacter sp. were found only by using ML primer pair, while the MLA primer pair detected sequences corresponding to Methanothrix sp.

A review of the operating parameters on the microbial fuel cell for wastewater treatment and electricity generation

Environmental and economic considerations suggest a more efficient and comprehensive use of biomass for bioenergy production. One of the most attractive technologies is the microbial fuel cell using the catabolic activity of microorganisms to generate electricity from organic matter. The microbial fuel cell (MFC) has operational benefits and higher performance than current technologies for producing energy from organic materials because it converts electricity from the substrate directly (at ambient temperature). However, MFCs are still not suitable for high energy demand due to practical limitations. The overall performance of an MFC depends on the electrode material, the reactor design, the operating parameters, substrates, and microorganisms. Furthermore, the optimization of the parameters will lead to the commercial development of this technology in the near future. The simultaneous effect of the parameters on each other (intensifier or attenuator) has also been investigated. The investigated parameters in this study include temperature, pH, flow rate and hydraulic retention time, mode, external resistance, and initial concentration.

Experimental investigation and mathematical modelling of batch and semi-continuous anaerobic digestion of cellulose at high concentrations and long residence times

In the context of the anaerobic digestion of slowly biodegradable substrates for energy and chemicals production, this study investigated the anaerobic digestion of cellulose without any chemical pre-treatments using open (undefined) mixed microbial cultures. The anaerobic conversion of cellulose was investigated in extended-length (run length in the range 518–734 days) batch and semi-continuous runs (residence time 20–80 days), at high cellulose concentration (20–40 g L−1), at temperatures of 25 and 35 °C. The maximum cellulose removal was 77% in batch (after 412 days) and 60% (at 80 days residence time) in semi-continuous experiments. In semi-continuous experiments, cellulose removal increased as the residence time increased however the cellulose removal rate showed a maximum (0.17 g L−1 day−1) at residence time 40–60 days. Both cellulose removal and removal rate decreased when cellulose concentration in the feed was increased from 20 to 40 g L−1. Liquid-phase products (ethanol and short chain organic acids) were only observed under transient conditions but not at the steady state of semi-continuous runs. Most of the observed results were well described by a mathematical model which included cellulose hydrolysis and growth on the produced glucose. The model provided insight into the physical phenomena behind the observed results.

Multivariable Robust Regulation of Alkalinities in Continuous Anaerobic Digestion Processes: Experimental Validation

A multivariable adaptive feedback control for highly uncertain continuous anaerobic digestion processes is proposed to regulate the volatile fatty acids (VFA) concentration, the strong ions concentrations, and the total and intermediate alkalinities. The multivariable control scheme includes a Luenberger observer to estimate both the unmeasured variables (i.e., VFA) and unknown microbial growth kinetics. The control approach is designed using an exponential Lyapunov function to resemble the typical exponential biological growth of the involved microbial consortia. Taking into account physicochemical equilibrium, alkalinities are represented as a function of the state variables. As a result, the control problem becomes a regulation problem on alkalinities, and in turn, a tracking control problem on the state variables, with two manipulated variables—the dilution rate and the feed rate of a strong alkali solution—while the state variables’ set-points are given as a function of pH. The implementation of this multivariable control scheme was experimentally tested and validated in a 0.982 m3 pilot plant treating agro-industrial wastewater and demonstrated to be robust in the face of unknown microbial growth kinetics. Results showed the potential for practical application and optimization of industrial digesters.

Co-digestion of palm oil mill effluent with chicken manure and crude glycerol: biochemical methane potential by monod kinetics

In Thailand, the palm oil industry produces a huge amount of palm oil mill effluent (POME), mostly used for electricity generation through biogas production. Co-digestion with other waste can further improve biogas yield and solve waste management problems. Most previous studies relied on biochemical methane potential (BMP) assay or batch co-digestion to obtain the optimal mixing ratio, ignoring the kinetic part or treat it for sole discussion of the results. This work directly uses mechanistic models based on Monod kinetics to describe the experimental results obtained from the co-digestion of POME (40 ml, BMP = 281.2 mlCH₄/gCODadded)) with chicken manure (CM) (0–50 g) and crude glycerol (Gly) (0–10 ml). The best mixing ratio between CM and POME was 5 gCM: 40 mlPOME (BMP = 276.9 mlCH₄/gCODadded). The best ratio for Gly and POME was 2 mlGly: 40 mlPOME (BMP = 211.9 mlCH₄/gCODadded). Adding Gly only 2 mlGly/40 mlPOME doubled the amount of biogas. Hence, crude glycerol is a good substrate for on-demand biogas output. The co-digestion increases the methane output but with a decreased yield. A multi-substrate Monod model was developed based on the levels of digestion difficulty. A partial-least squared fitting was used to estimate its main parameters. All parameters included in the model passed the significant tests at a 95% confidence level. The model can describe the experimental results very well, predict observable state variables of batch co-digestion, and allow a simple extension for continuous co-digestion dynamics. A limited continuous experiment was conducted to confirm the applicability of the model parameters of POME digestion obtained from BMP tests to predict a continuous AD. The results show good potential but must be carefully interpreted. It is generally possible and practical to directly obtain design and operational parameters from BMP assays based on only accumulated biogas curves and initial and final COD/VS.

The effect of brewery spent grain application on biogas yields and kinetics in co-digestion with sewage sludge

The present study examines the effect of introducing dried brewery spent grain (BSG), known as the main solid by-product of the brewery industry on biogas yields and kinetics in co-digestion with sewage sludge (SS). The experiment was conducted in semi-continuous anaerobic reactors (supplied once a day) operating under mesophilic conditions (35°C) at different hydraulic retention times (HRT) of 18 and 20 d. In co-digestion runs, the BSG mass to the feed volume ratio was constant and maintained 1:10.The results indicated that the addition of BSG did not influence the biogas production, by comparison with SS mono-digestion (control run). At HRT of 18 d, in the co-digestion run, the average methane yield was 0.27 m³ kg/VS_added, while in the control run the higher value of 0.29 m³ kg/VS_addedwas observed. However, there was no difference in terms of statistical significance. At HRT of 20 d, the methane yield was 0.21 m³ kg/VS_added for both mono- and co-digestion runs. In the BSG presence, the decrease in kinetic constant values was observed. As compared to SS mono-digestion, reductions by 21 and 35% were found at HRT of 20 and 18 d, respectively. However, due to the supplementation of the feedstock with BSG rich in organic compounds, the significantly enhanced energy profits were achieved with the highest value of approx. 40% and related to the longer HRT of 20 d. Importantly, the mono- and co-digestion process proceeded in stable manner. Therefore, the anaerobic co-digestion of SS and BSG might be considered as a cost-effective solution that could contribute to the energy self-efficiency of wastewater treatment plants (WWTPs) and sustainable waste management for breweries.

Microbially induced carbonate precipitation via methanogenesis pathway by a microbial consortium enriched from activated anaerobic sludge

AIMS

Various applications of microbially induced carbonate precipitation (MICP) has been proposed. However, most studies use cultured pure strains to obtain MICP, ignoring advantages of microbial consortia. The aims of this study were to: (i) test the feasibility of a microbial consortium to produce MICP; (ii) identify functional micro-organisms and their relationship; (iii) explain the MICP mechanism; (iv) propose a way of applying the MICP technique to soil media.

METHODS AND RESULTS

Anaerobic sludge was used as the source of the microbial consortium. A laboratory anaerobic sequencing batch reactor and beaker were used to perform precipitation experiment. The microbial consortium produced MICP with an efficiency of 96·6%. XRD and SEM analysis showed that the precipitation composed of different-size calcite crystals. According to high-throughput 16S rRNA gene sequencing, the functional micro-organisms included acetogenic bacteria, acetate-oxidizing bacteria and archaea Methanosaeta and Methanobacterium beijingense. The methanogenesis acetate degradation provides dissolved inorganic carbon and increases pH for MICP. A series of reactions catalysed by many enzymes and cofactors of methanogens and acetate-oxidizers are involved in the acetate degradation.

CONCLUSION

This work demonstrates the feasibility of using the microbial consortium to achieve MICP from an experimental and theoretical perspective.

SIGNIFICANCE AND IMPACT OF THE STUDY

A method of applying the microbial-consortium MICP to soil media is proposed. It has the advantages of low cost, low environmental impact, treatment uniformity and less limitations from natural soils. This method could be used to improve mechanical properties, plug pores and fix harmful elements of soil media, etc.

Not Just Numbers: Mathematical Modelling and Its Contribution to Anaerobic Digestion Processes

Mathematical modelling of bioprocesses has a long and notable history, with eminent contributions from fields including microbiology, ecology, biophysics, chemistry, statistics, control theory and mathematical theory. This richness of ideas and breadth of concepts provide great motivation for inquisitive engineers and intrepid scientists to try their hand at modelling, and this collaboration of disciplines has also delivered significant milestones in the quality and application of models for both theoretical and practical interrogation of engineered biological systems. The focus of this review is the anaerobic digestion process, which, as a technology that has come in and out of fashion, remains a fundamental process for addressing the global climate emergency. Whether with conventional anaerobic digestion systems, biorefineries, or other anaerobic technologies, mathematical models are important tools that are used to design, monitor, control and optimise the process. Both highly structured, mechanistic models and data-driven approaches have been used extensively over half a decade, but recent advances in computational capacity, scientific understanding and diversity and quality of process data, presents an opportunity for the development of new modelling paradigms, augmentation of existing methods, or even incorporation of tools from other disciplines, to ensure that anaerobic digestion research can remain resilient and relevant in the face of emerging and future challenges.

Beneficial effects of intermittent feedstock management on biogas and methane production

Intermittent supply of easily degradable carbohydrates can be used for on-demand biogas production. The study tested the effects of splitting feeding portions of sugar beet silage (S) on biogas production rates and total yield, respectively and if methane production rates follow those ones of biogas. Four experimental AD reactors were operated for 117 days at organic loading rates of 2.0 kg_VS m^-3 d^-1 and VS ratios of maize silage (M) to S of 3:1. While M was supplied hourly (h₀-h₁₂), reactors differed only regarding the intermittent S supply, provided at once (h₀), twice (h₀, h₁) and three times (h₀, h₁, h₂) per twelve-hour observation period. Biogas and methane production rates rose simultaneously after S supply and lasted depending on S intakes. Biogas and methane yields were significantly increased at S given once and twice per period. Appropriate feedstock management can thus influence production rates and increase biogas and methane yields.

Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review

Anaerobic digestion (AD) is a well-established technology used for producing biogas or biomethane alongside the slurry used as biofertilizer. However, using a variety of wastes and residuals as substrate and mixed cultures in the bioreactor makes AD as one of the most complicated biochemical processes employing hydrolytic, acidogenic, hydrogen-producing, acetate-forming bacteria as well as acetoclastic and hydrogenoclastic methanogens. Hydrogen and volatile fatty acids (VFAs) including acetic, propionic, isobutyric, butyric, isovaleric, valeric and caproic acid and other carboxylic acids such as succinic and lactic acids are formed as intermediate products. As these acids are important precursors for various industries as mixed or purified chemicals, the AD process can be bioengineered to produce VFAs alongside hydrogen and therefore biogas plants can become biorefineries. The current review paper provides the theory and means to produce and accumulate VFAs and hydrogen, inhibit their conversion to methane and to extract them as the final products. The effects of pretreatment, pH, temperature, hydraulic retention time (HRT), organic loading rate (OLR), chemical methane inhibitions, and heat shocking of the inoculum on VFAs accumulation, hydrogen production, VFAs composition, and the microbial community were discussed. Furthermore, this paper highlights the possible techniques for recovery of VFAs from the fermentation media in order to minimize product inhibition as well as to supply the carboxylates for downstream procedures.

On the Effect of Aqueous Ammonia Soaking Pre-Treatment on Continuous Anaerobic Digestion of Digested Swine Manure Fibers

(1) Background: The continuously increasing demand for renewable energy sources renders anaerobic digestion as one of the most promising technologies for renewable energy production. Due to the animal production intensification, manure is being used as the primary feedstock for most biogas plants. Their economical profitable operation, however, relies on increasing the methane yield from the solid fraction of manure, which is not so easily degradable. The solid fraction after anaerobic digestion, the so-called digested fibers, consists mainly of hardly biodegradable material and comes at a lower mass per unit volume of manure compared to the solid fraction before anaerobic digestion. Therefore, investigation on how to increase the biodegradability of digested fibers is very relevant. So far, Aqueous Ammonia Soaking (AAS), has been successfully applied on digested fibers separated from the effluent of a manure-fed, full-scale anaerobic digester to enhance their methane productivity in batch experiments. (2) Methods: In the present study, continuous experiments at a mesophilic (38 °C) CSTR-type anaerobic digester fed with swine manure first and a mixture of manure with AAS-treated digested fibers in the sequel, were performed. Anaerobic Digestion Model 1 (ADM1) previously fitted on manure fed digester was used in order to assess the effect of the addition of AAS-pre-treated digested manure fibers on the kinetics of anaerobic digestion process. (3) Results and Conclusions: The methane yield of AAS-treated digested fibers under continuous operation was 49–68% higher than that calculated in batch experiments in the past. It was found that AAS treatment had a profound effect mainly on the disintegration/hydrolysis rate of particulate carbohydrates. Comparison of the data obtained in the present study with the data obtained with AAS-pre-treated raw manure fibers in the past revealed that hydrolysis kinetics after AAS pre-treatment were similar for both types of biomasses.

Effect of Bioaugmentation on Biogas Yields and Kinetics in Anaerobic Digestion of Sewage Sludge

Bioaugmentation with a mixture of microorganisms (Bacteria and Archaea) was applied to improve the anaerobic digestion of sewage sludge. The study was performed in reactors operating at a temperature of 35 °C in semi-flow mode. Three runs with different doses of bioaugmenting mixture were conducted. Bioaugmentation of sewage sludge improved fermentation and allowed satisfactory biogas/methane yields and a biodegradation efficiency of more than 46%, despite the decrease in hydraulic retention time (HRT) from 20 d to 16.7 d. Moreover, in terms of biogas production, the rate constant k increased from 0.071 h⁻¹ to 0.087 h⁻¹ as doses of the bioaugmenting mixture were increased, as compared to values of 0.066 h⁻¹ and 0.069 h⁻¹ obtained with sewage sludge alone. Next-generation sequencing revealed that Cytophaga sp. predominated among Bacteria in digesters and that the hydrogenotrophic methanogen Methanoculleus sp. was the most abundant genus among Archaea.

Biochemical methane potential (BMP) tests: Reducing test time by early parameter estimation

Biochemical methane potential (BMP) test is a key analytical technique to assess the implementation and optimisation of anaerobic biotechnologies. However, this technique is characterised by long testing times (from 20 to >100days), which is not suitable for waste utilities, consulting companies or plants operators whose decision-making processes cannot be held for such a long time. This study develops a statistically robust mathematical strategy using sensitivity functions for early prediction of BMP first-order model parameters, i.e. methane yield (B₀) and kinetic constant rate (k). The minimum testing time for early parameter estimation showed a potential correlation with the k value, where (i) slowly biodegradable substrates (k≤0.1d^-1) have a minimum testing times of ≥15days, (ii) moderately biodegradable substrates (0.1<k<0.2d^-1) have a minimum testing times between 8 and 15 days, and (iii) rapidly biodegradable substrates (k≥0.2d^-1) have testing times lower than 7days.

Modeling de novo granulation of anaerobic sludge

BACKGROUND

A unique combination of mechanical, physiochemical and biological forces influences granulation during processes of anaerobic digestion. Understanding this process requires a systems biology approach due to the need to consider not just single-cell metabolic processes, but also the multicellular organization and development of the granule.

RESULTS

In this computational experiment, we address the role that physiochemical and biological processes play in granulation and provide a literature-validated working model of anaerobic granule de novo formation. The agent-based model developed in a cDynoMiCs simulation environment successfully demonstrated a de novo granulation in a glucose fed system, with the average specific methanogenic activity of 1.11 ml C H ₄/g biomass and formation of a 0.5 mm mature granule in 33 days. The simulated granules exhibit experimental observations of radial stratification: a central dead core surrounded by methanogens then encased in acidogens. Practical application of the granulation model was assessed on the anaerobic digestion of low-strength wastewater by measuring the changes in methane yield as experimental configuration parameters were systematically searched.

CONCLUSIONS

In the model, the emergence of multicellular organization of anaerobic granules from randomly mixed population of methanogens and acidogens was observed and validated. The model of anaerobic de novo granulation can be used to predict the morphology of the anaerobic granules in a alternative substrates of interest and to estimate methane potential of the resulting microbial consortia. The study demonstrates a successful integration of a systems biology approach to model multicellular systems with the engineering of an efficient anaerobic digestion system.

Piezo-tolerant natural gas-producing microbes under accumulating pCO2

BACKGROUND

It is known that a part of natural gas is produced by biogenic degradation of organic matter, but the microbial pathways resulting in the formation of pressurized gas fields remain unknown. Autogeneration of biogas pressure of up to 20 bar has been shown to improve the quality of biogas to the level of biogenic natural gas as the fraction of CO₂ decreased. Still, the pCO₂ is higher compared to atmospheric digestion and this may affect the process in several ways. In this work, we investigated the effect of elevated pCO₂ of up to 0.5 MPa on Gibbs free energy, microbial community composition and substrate utilization kinetics in autogenerative high-pressure digestion.

RESULTS

In this study, biogas pressure (up to 2.0 MPa) was batch-wise autogenerated for 268 days at 303 K in an 8-L bioreactor, resulting in a population dominated by archaeal Methanosaeta concilii, Methanobacterium formicicum and Mtb. beijingense and bacterial Kosmotoga-like (31% of total bacterial species), Propioniferax-like (25%) and Treponema-like (12%) species. Related microorganisms have also been detected in gas, oil and abandoned coal-bed reservoirs, where elevated pressure prevails. After 107 days autogeneration of biogas pressure up to 0.50 MPa of pCO₂, propionate accumulated whilst CH₄ formation declined. Alongside the Propioniferax-like organism, a putative propionate producer, increased in relative abundance in the period of propionate accumulation. Complementary experiments showed that specific propionate conversion rates decreased linearly from 30.3 mg g^-1 VS_added day^-1 by more than 90% to 2.2 mg g^-1 VS_added day^-1 after elevating pCO₂ from 0.10 to 0.50 MPa. Neither thermodynamic limitations, especially due to elevated pH₂, nor pH inhibition could sufficiently explain this phenomenon. The reduced propionate conversion could therefore be attributed to reversible CO₂-toxicity.

CONCLUSIONS

The results of this study suggest a generic role of the detected bacterial and archaeal species in biogenic methane formation at elevated pressure. The propionate conversion rate and subsequent methane production rate were inhibited by up to 90% by the accumulating pCO₂ up to 0.5 MPa in the pressure reactor, which opens opportunities for steering carboxylate production using reversible CO₂-toxicity in mixed-culture microbial electrosynthesis and fermentation.Graphical abstractThe role of pCO₂ in steering product formation in autogenerative high pressure digestion.

A Novel Mesophilic Anaerobic Digestion System for Biogas Production and In Situ Methane Enrichment from Coconut Shell Pyroligneous

A novel mesophilic anaerobic digestion process with detoxification-treated coconut shell pyroligneous was established, exhibiting an effective advantage in biogas production. The pyroligneous collected contained 166.2 g l(-1) acetic acid, indicating great potential for biogas production. Detoxification was an effective way of simultaneously enriching biodegradable ingredients and removing inhibitors (mainly as phenols and organic acids) for digestion process. The digestion process lasted 96 h and fermentation characteristics (chemical oxygen demand (COD) removal ratio, volatile fatty acid (VFA) consumptions, pH, total gas, methane yield, and phenol removal efficiency) were measured. The experiments successfully explored the optimum detoxification parameters, oxidized with 10 % H2O2 followed by overliming, and demonstrated 89.3 % COD removal, 91.4 % methane content, 0.305 LCH4/g COD removed CH4 yield, and 88.81 % phenol removal ratio. This study provided clues to overcome the negative effects of inhibitors in pyroligneous on biogas production. The findings could contribute to significant process in detoxified pretreatment of pyroligneous and develop an economically feasible technology for treating pyroligneous after producing charcoal.

Methane Production from Rice Straw Hydrolysate Treated with Dilute Acid by Anaerobic Granular Sludge

The traditional anaerobic digestion process of straw to biogas faces bottlenecks of long anaerobic digestion time, low digestion rate, less gas production, etc., while straw hydrolysate has the potential to overcome these drawbacks. In this study, the dilute sulphuric acid-treated hydrolysate of rice straw (DSARSH) containing high sulfate was firstly proved to be a feasible substrate for methane production under mesophilic digestion by granular sludge within a short digestion time. Batch anaerobic digestion process was operated under different initial chemical oxygen demand (COD) values at temperature of 37 °C with the pH of 8.5. Among the initial COD values ranging from 3000 to 11,000 mg/L, 5000 mg/L was proved to be the most appropriate considering high COD removal efficiency (94.17 ± 1.67 %), CH4 content (65.52 ± 3.12 %), and CH4 yield (0.346 ± 0.008 LCH4/g COD removed) within 120 h. Furthermore, when the studied system operated at the initial COD of 5000 mg/L, the sulfate removal ratio could reach 56.28 %.

Mechanism, kinetics and microbiology of inhibition caused by long-chain fatty acids in anaerobic digestion of algal biomass

BACKGROUND

Oleaginous microalgae contain a high level of lipids, which can be extracted and converted to biofuel. The lipid-extracted residue can then be further utilized through anaerobic digestion to produce biogas. However, long-chain fatty acids (LCFAs) have been identified as the main inhibitory factor on microbial activity of anaerobic consortium. In this study, the mechanism of LCFA inhibition on anaerobic digestion of whole and lipid-extracted algal biomass was investigated with a range of calcium concentrations against various inoculum to substrate ratios as a means to alleviate the LCFA inhibition.

RESULTS

Whole algal biomass of Nannochloropsis salina represents high lipid content algal biomass while lipid-extracted residue represents its low lipid counterpart. The anaerobic digestion experiments were conducted in a series of serum bottles at 35 °C for 20 days. A kinetic model, considering LCFA inhibition on hydrolysis, acidogenesis as well as methanogenesis steps, was developed from the observed phenomenon of inhibition factors as a function of the LCFA concentration and specific biomass content or calcium concentration. The results showed that inoculum to substrate ratio had a stronger effect on biogas production than calcium, and calcium had no effect on biogas production when inoculum concentration was extremely low. The microbial community analysis by high-throughput Illumina Miseq sequencing indicated that diversity of both bacterial and methanogenic communities decreased with elevation of lipid concentration. Hydrolytic bacteria and aceticlastic methanogens dominated bacterial and archaea communities, respectively, in both high and low LCFA concentration digesters.

CONCLUSIONS

This study demonstrated that inoculum concentration has a more significant effect on alleviating LCFA inhibition than calcium concentration, while calcium only played a role when inoculum concentration met a threshold level. The model revealed that each functional microbial group was subject to different levels of LCFA inhibition. Although methanogens were the most susceptible microbes to LCFA inhibition, the inhibition factor for hydrolytic bacteria was more highly affected by inoculum concentration. The microbial community analysis indicated that the bacterial community was affected more than the methanogenic community by high LCFAs concentration. Syntrophic acetogens were sensitive to high LCFA concentrations and thus showed a decreased abundance in such an environment. Graphical abstractProposed mechanism of calcium mitigated LCFA inhibition.

Modelling wastewater treatment in a submerged anaerobic membrane bioreactor

Mathematical modelling has been widely applied to membrane bioreactor (MBRs) processes. However, to date, very few studies have reported on the application of the anaerobic digestion model N.1 (ADM1) to anaerobic membrane processes. The aim of this study was to evaluate the applicability of the ADM1 to a submerged anaerobic MBR (SAMBR) treating simulated industrial wastewater composed of cheese whey and sucrose. This study demonstrated that the biological processes involved in SAMBRs can be modelled by using the ADM1. Moreover, the results showed that very few modifications of the parameters describing the ADM1 were required to reasonably fit the experimental data. In particular, adaptation to the specific conditions of the coefficients describing the wastewater characterisation and the reduction of the hydrolysis rate of particulate carbohydrate (khyd,ch) from 0.25 d(-1) (as suggested by the ADM1 for high-rate mesophilic reactors) to 0.13 d(-1) were required to fit the experimental data.

Simulation of anaerobic digestion processes using stochastic algorithm

BACKGROUND

The Anaerobic Digestion (AD) processes involve numerous complex biological and chemical reactions occurring simultaneously. Appropriate and efficient models are to be developed for simulation of anaerobic digestion systems. Although several models have been developed, mostly they suffer from lack of knowledge on constants, complexity and weak generalization. The basis of the deterministic approach for modelling the physico and bio-chemical reactions occurring in the AD system is the law of mass action, which gives the simple relationship between the reaction rates and the species concentrations. The assumptions made in the deterministic models are not hold true for the reactions involving chemical species of low concentration. The stochastic behaviour of the physicochemical processes can be modeled at mesoscopic level by application of the stochastic algorithms.

METHOD

In this paper a stochastic algorithm (Gillespie Tau Leap Method) developed in MATLAB was applied to predict the concentration of glucose, acids and methane formation at different time intervals. By this the performance of the digester system can be controlled. The processes given by ADM1 (Anaerobic Digestion Model 1) were taken for verification of the model.

RESULTS

The proposed model was verified by comparing the results of Gillespie's algorithms with the deterministic solution for conversion of glucose into methane through degraders. At higher value of 'τ' (timestep), the computational time required for reaching the steady state is more since the number of chosen reactions is less. When the simulation time step is reduced, the results are similar to ODE solver.

CONCLUSION

It was concluded that the stochastic algorithm is a suitable approach for the simulation of complex anaerobic digestion processes. The accuracy of the results depends on the optimum selection of tau value.

Factors Affecting Process Temperature and Biogas Production in Small-scale Rural Biogas Digesters in Winter in Northern Vietnam

This study investigated the main factors influencing digester temperature and methods to reduce heat losses during the cold season in the subtropics. Four composite digesters (two insulated and two uninsulated) were buried underground to measure their internal temperature (°C) at a depth of 140 cm and 180 cm, biogas production and methane (CH4) concentration in biogas from August to February. In parallel the temperature of the air (100 cm above ground), in the slurry mixing tank and in the soil (10, 100, 140, and 180 cm depth) was measured by thermocouple. The influent amount was measured daily and the influent chemical composition was measured monthly during the whole experimental period. Seasonal variations in air temperature significantly affected the temperature in the soil, mixing tank and digester. Consequently, biogas production, which is temperature dependent, was influenced by the season. The main factors determining the internal temperature in the digesters were insulation with Styrofoam, air temperature and temperature of slurry in the mixing tank. Biogas production is low due to the cold climate conditions in winter in Northern Vietnam, but the study proved that storing slurry in the mixing tank until its temperature peak at around 14:00 h will increase the temperature in the digester and thus increase potential biogas production. Algorithms are provided linking digester temperature to the temperature of slurry in the mixing tank.

Absolute quantification of individual biomass concentrations in a methanogenic coculture

Identification of individual biomass concentrations is a crucial step towards an improved understanding of anaerobic digestion processes and mixed microbial conversions in general. The knowledge of individual biomass concentrations allows for the calculation of biomass specific conversion rates which form the basis of anaerobic digestion models. Only few attempts addressed the absolute quantification of individual biomass concentrations in methanogenic microbial ecosystems which has so far impaired the calculation of biomass specific conversion rates and thus model validation. This study proposes a quantitative PCR (qPCR) approach for the direct determination of individual biomass concentrations in methanogenic microbial associations by correlating the native qPCR signal (cycle threshold, Ct) to individual biomass concentrations (mg dry matter/L). Unlike existing methods, the proposed approach circumvents error-prone conversion factors that are typically used to convert gene copy numbers or cell concentrations into actual biomass concentrations. The newly developed method was assessed and deemed suitable for the determination of individual biomass concentrations in a defined coculture of Desulfovibrio sp. G11 and Methanospirillum hungatei JF1. The obtained calibration curves showed high accuracy, indicating that the new approach is well suited for any engineering applications where the knowledge of individual biomass concentrations is required.

Integration of sewage sludge digestion with advanced biofuel synthesis

Sewage sludge rich in carbohydrates and other nutrients could be a good feedstock for fuel/chemical production. In this study, fungal and engineered bacterial cultivations were integrated with a modified anaerobic digestion to accumulate fatty acids on sewage sludge. The anaerobic digestion was first adjusted to enable acetogenic bacteria to accumulate acetate. A fungus (Mortierella isabellina) and an engineered bacterium (Escherichia coli created by optimizing acetate utilization and fatty acid biosynthesis as well as overexpressing a regulatory transcription factor fadR) were then cultured on the acetate solution to accumulate fatty acids. The engineered bacterium had higher fatty acid yield and titer than the fungus. Both medium- and long-chain fatty acids (C12:0-C18:0) were produced by the engineered bacterium, while the fungus mainly synthesized long-chain fatty acids (C16:0-C18:3). This study demonstrated a potential path that combines fungus or engineered bacterium with anaerobic digestion to achieve simultaneous organic waste treatment and advanced biofuel production.

Model selection, identification and validation in anaerobic digestion: a review

Anaerobic digestion enables waste (water) treatment and energy production in the form of biogas. The successful implementation of this process has lead to an increasing interest worldwide. However, anaerobic digestion is a complex biological process, where hundreds of microbial populations are involved, and whose start-up and operation are delicate issues. In order to better understand the process dynamics and to optimize the operating conditions, the availability of dynamic models is of paramount importance. Such models have to be inferred from prior knowledge and experimental data collected from real plants. Modeling and parameter identification are vast subjects, offering a realm of approaches and methods, which can be difficult to fully understand by scientists and engineers dedicated to the plant operation and improvements. This review article discusses existing modeling frameworks and methodologies for parameter estimation and model validation in the field of anaerobic digestion processes. The point of view is pragmatic, intentionally focusing on simple but efficient methods.

A comprehensive microbial insight into two-stage anaerobic digestion of food waste-recycling wastewater

Microbial community structures were assessed in a two-stage anaerobic digestion system treating food waste-recycling wastewater. The reactors were operated for 390 d at 10 different hydraulic retention times (HRTs) ranging from 25 to 4 d. Stable operation was achieved with the overall chemical oxygen demand (COD) removal efficiency of 73.0-85.9% at organic loading rate of up to 35.6 g COD/L·d. Performance of the acidogenic reactors, however, changed significantly during operation. This change coincided with transition of the bacterial community from one dominated by Aeriscardovia- and Lactobacillus amylovorus-related species to one dominated by Lactobacillus acetotolerans- and Lactobacillus kefiri-like organisms. In methanogenic reactors, the microbial community structures also changed at this stage along with the shift from Methanoculleus- to Methanosarcina-like organisms. This trend was confirmed by the non-metric multidimensional scaling joint plot of microbial shifts along with performance parameters. These results indicated that the overall process performance was relatively stable compared to the dynamic changes in the microbial structures and the acidogenic performance.

Mathematical modelling of carbohydrate degradation by human colonic microbiota

The human colon is an anaerobic ecosystem that remains largely unexplored as a result of its limited accessibility and its complexity. Mathematical models can play a central role for a better insight into its dynamics. In this context, this paper presents the development of a mathematical model of carbohydrate degradation. Our aim was to provide an in silico approach to contribute to a better understanding of the fermentation patterns in such an ecosystem. Our mathematical model is knowledge-based, derived by writing down mass-balance equations. It incorporates physiology of the intestine, metabolic reactions and transport phenomena. The model was used to study various nutritional scenarios and to assess the role of the mucus on the system behavior. Model simulations provided an adequate qualitative representation of the human colon. Our model is complementary to experimental studies on human colonic fermentation, which, of course, is not meant to replace. It may be helpful to gain insight on questions that are still difficult to elucidate by experimentation and suggest future experiments.

Effect of corrosion of steel elements on the treatment of dairy wastewater in a UASB reactor

Experiments were performed in parallel using two laboratory upflow anaerobic sludge blanket (UASB) reactors. One of the two reactors was packed with spiral elements made of steel wire with 48% iron content in order to examine the influence of the steel elements on the chemical oxygen demand (COD) and efficiency of phosphorus removal from synthetically prepared dairy wastewater. A strong relationship was found between anaerobic corrosion and efficiency of phosphorus removal. Phosphorus removal in the reactor packed with steel elements was between 16.4% and 64.4% higher than without the steel elements present. The anaerobic corrosion process improved COD removal efficiency by 1.0-3.1%, which was statistically significant. When steel elements were present the methane content of the biogas was increased by 6.7%. Increasing the organic loading rate had a strong effect on the anaerobic efficiency of the dairy wastewater treatment.

Biogas production: current state and perspectives

Anaerobic digestion of energy crops, residues, and wastes is of increasing interest in order to reduce the greenhouse gas emissions and to facilitate a sustainable development of energy supply. Production of biogas provides a versatile carrier of renewable energy, as methane can be used for replacement of fossil fuels in both heat and power generation and as a vehicle fuel. For biogas production, various process types are applied which can be classified in wet and dry fermentation systems. Most often applied are wet digester systems using vertical stirred tank digester with different stirrer types dependent on the origin of the feedstock. Biogas is mainly utilized in engine-based combined heat and power plants, whereas microgas turbines and fuel cells are expensive alternatives which need further development work for reducing the costs and increasing their reliability. Gas upgrading and utilization as renewable vehicle fuel or injection into the natural gas grid is of increasing interest because the gas can be used in a more efficient way. The digestate from anaerobic fermentation is a valuable fertilizer due to the increased availability of nitrogen and the better short-term fertilization effect. Anaerobic treatment minimizes the survival of pathogens which is important for using the digested residue as fertilizer. This paper reviews the current state and perspectives of biogas production, including the biochemical parameters and feedstocks which influence the efficiency and reliability of the microbial conversion and gas yield.

Archaea diversity within a commercial biogas plant utilizing herbal biomass determined by 16S rDNA and mcrA analysis

AIMS

The Archaea diversity was evaluated in an agricultural biogas plant supplied with cattle liquid manure and maize silage under mesophilic conditions.

METHODS AND RESULTS

Two different genes (16S rRNA; methyl-coenzyme-M-reductase, MCR) targeted by three different PCR primer sets were selected and used for the construction of three clone libraries comprising between 104 and 118 clones. The clone libraries were analysed by restriction fragment polymorphism (RFLP). Between 11 and 31 operational taxonomic units (OTUs) were detected and assigned to orders Methanomicrobiales, Methanosarcinales and Methanobacteriales. Over 70% of all Archaea OTUs belong to the order Methanomicrobiales which mostly include hydrogenotrophic methanogens. Acetotrophic methanogens were detected in minor rates. Similar relative values were obtained by a quantitative real-time PCR analysis.

CONCLUSIONS

The results implied that in this biogas plant the most of the methane formation resulted from the conversion of H(2) and CO(2).

SIGNIFICANCE AND IMPACT OF THE STUDY

This study reports, for the first time, a molecular analysis of the archaeal community in this type of agricultural biogas plants. Therein the hydrogenotrophic methanogenesis seems to be the major pathway of methane formation. These results are in contrast with the common thesis that in biogas fermentations the primary substrate for methanogenesis is acetate.